What do you think about a callback function where the user could
define more precisely how formatting is done (like adding symbols for

percentages etc.)

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I really like the idea and your proof of concept. Thanks for coming up
with this!
However I'd suggest to implement this more general and flexible in some
way. Maybe we could create different general formatting functions which
apply for example to percentage.
Also, I am not sure if it would be better to add these functions
post-first-release.